48 seconds, Consider what happens between one time that Ben and
Sally meet and the next. During this period Ben runs 30 (seconds)/80 (seconds/lap) = 3/8 of a lap. So Sally runs 5/8 lap in the same
time. Her speed is therefore 5/8 (laps)/ 30(seconds) which amounts
to 1 (lap)/ 48 (seconds).
